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The neurochemistry of schizophrenia
Research aspects
The neurochemistry of schizophrenia
What’s new? • The genes implicated as risk factors in schizophrenia are increasingly found to influence neuronal and synaptic neurochemistry
Gavin P Reynolds
• Research has focused particularly on glutamatergic systems, and drugs are now in development that directly act on glutamate neurotransmission. If they prove successful in the clinic, this integration of genetics, neurochemistry, and pharmacology may have brought about a move away from dopamine antagonism in antipsychotic treatment
Abstract For more than 30 years much of the focus of neurochemical research in schizophrenia has been on the dopamine hypothesis, although serotonin systems may also be dysfunctional. Certainly, the primary action of antipsychotic drugs is to diminish dopamine D2 receptor-mediated neurotransmission. Although there is little indication of primary disturbances in dopamine (or serotonin) neurotransmission in the schizophrenia, recent functional neuro-imaging studies have demonstrated an increase in stimulated release of dopamine in the brain of patients with schizophrenia. It seems likely that this neurochemical correlate of positive symptoms might be secondary to disturbances in other neurotransmitter systems. Evidence from in vivo imaging and post-mortem studies of the brain in schizophrenia, as well as from experimental models, points to deficits of γ-aminobutyric acid (GABA)-containing neurons, and dysfunction of glutamate-containing neurons, in the cortex and elsewhere. Such regionally specific neuronal abnormalities probably underlie negative features and cognitive deficits, as well as contributing to a disinhibition of subcortical dopamine. Experimental models suggest that GABAergic deficits, perhaps of developmental origin, could result in progressive damage to other neuronal systems. Several of the recently identified genetic risk factors for schizophrenia also influence neurotransmitter and synaptic function, with some convergence on glutamate. This is providing new targets for antipsychotic drug treatment.
observations of neurochemical disturbances in living brains have been obtained using magnetic resonance spectroscopy (MRS), and positron and single-photon emission (computed) tomography (PET and SPET/SPECT respectively). The application of molecular genetics has also provided clues as to the underlying aetiologies and pathological processes in schizophrenia. Despite these technological developments, there are nevertheless inconsistencies in neurochemical findings in the disease. These problems may be due in part to the effects of drug treatment, as well as differences between patient cohorts (diagnosis in schizophrenia has not always been reliable, and symptom profile can vary enormously between individuals). The absence of a single core syndrome, and the multiplicity of proposed aetiological factors, indicates that schizophrenia is likely to be a complex disorder in which brain pathology may well differ between individuals owing to differences in pathogenic mechanisms. Early hypotheses Initial neurochemical hypotheses of brain dysfunction in schizophrenia assumed a disturbance of brain biochemistry. This approach originated in the observation that psychosis in humans, and equivalent bizarre behaviours in animals, could be induced by certain ‘psychotogenic’ drugs. It was assumed that the metabolic and pharmacological effects of these drugs might reflect the underlying biochemical abnormality in schizophrenia. Two hypotheses of the early 1950s involved excessive transmethylation and serotonin deficiency. It was suggested that neuro-active drugs with similarities to neurotransmitters, such as mescaline and dimethyltryptamine, were proposed to be formed by an overactive methylation process. Structurally similar psychotogenic compounds, such as lysergic acid diethylamide (LSD), were found to have effects via serotonin (5-hydroxytryptamine; 5-HT) receptors, and hence a deficiency of 5-HT neurotransmission was proposed. In retrospect, it is easy to identify limitations of these hypotheses, one being the inadequacy of the drug-induced ‘model psychosis’ in which distortions of reality and visual hallucinations are a major feature, rather than the auditory hallucinations and delusions more commonly seen in schizophrenia.
Keywords antipsychotic drugs; dopamine; dopamine receptors; GABA; glutamate; N-acetylaspartate; neuro-imaging; neurotransmitter
Methods and models The first approaches to understanding the pathophysiology of psychiatric disorders were chemical investigations of body fluids and, occasionally, brain tissue. These were under way long before the emergence of modern psychiatry, and for over 200 years have reflected methodological development, starting with the application of quantitative chemical analysis. The neurochemical study of the brain in schizophrenia depends on technological advance as much now as it did two centuries ago. Recently, exciting
Gavin P Reynolds PhD is Professor of Neuroscience in the Division of Psychiatry and Neuroscience at Queen’s University, Belfast, Northern Ireland, UK, and President of the British Association for Psychopharmacology. His main research interests are the pathology of neurotransmitter systems in schizophrenia and the mechanisms and pharmacogenetics of antipsychotic drug action. Conflicts of interest: none declared.
PSYCHIATRY 7:10
5-HT neurotransmission Nevertheless there remains an interest in the role of 5-HT in schizophrenia. Circumstantial support is provided by the atypical 425
© 2008 Elsevier Ltd. All rights reserved.
Research aspects
antipsychotic drugs, which all exhibit antagonism for the 5-HT2A receptor subtype; this provides a putative mechanism, not only for their low propensity to cause extrapyramidal side effects, but also for some efficacy in ameliorating the negative features of schizophrenia.1 More direct involvement is demonstrated by post-mortem and some neuro-imaging studies that have shown decrements in 5-HT2A receptors, and increases in 5-HT1A receptors, in the brain in schizophrenia. This is likely to reflect disturbances in 5-HT neurotransmission, although it is far from clear how these neurochemical abnormalities might originate in, or be secondary to, other neuronal pathologies such as those of γaminobutyric acid (GABA) and glutamate systems (see below). Nevertheless, changes in 5-HT function are perhaps unsurprising, given its central role in depression, a frequent symptom in schizophrenia, and the sensitivity of 5-HT systems to stress, an inevitable experience for many patients. Other neurochemical hypotheses have been proposed, based on aberrant neurotransmitter function (e.g. implicating noradrenaline or enkephalin), or metabolism, such as diminished monoamine oxidase activity. However, none of these has survived careful scrutiny and testing.
Dopamine pathways in the brain Physiological effects
Nigrostriatal
Involved in motor control; blockade of striatal dopamine D2 receptors produces extrapyramidal side effects; also involved in some cognitive circuits Blockade of limbic D2 receptors may alleviate positive symptoms of schizophrenia; limbic dopamine is also involved in reward and addiction Dopaminergic mechanisms in the frontal cortex modulate cognitive function, primarily via D1 receptors; COMT activity influences cortical synaptic dopamine Projection from hypothalamus controls pituitary hormonal secretion; blockade of D2 receptors disinhibits secretion of prolactin
Mesolimbic
Mesocortical
Tubero-infundibular
Table 1
The dopamine hypothesis Dopamine receptors What has survived the test of time, albeit with some elaboration, is the dopamine hypothesis of schizophrenia. This was first based on the ability of amphetamine, which stimulates dopamine release, to induce a psychosis with schizophreniform features. Further support came from the finding that almost all antipsychotic drugs are effective antagonists of the D2 subtype(s) of dopamine receptor, this antagonism correlating closely with clinical dosage. Subsequently, the finding of more D2 receptors in post-mortem brain from schizophrenic patients led to the hypothesis that the increase in D2 receptors resulted in the positive symptoms of schizophrenia. However, as an up-regulation of D2 receptors is seen in animals after chronic administration of antipsychotic drugs, it seemed likely that the increase in schizophrenia is a consequence of drug treatment and unrelated to the disease process. Most post-mortem and imaging studies now conclude that D2 receptors are not increased in drug-free schizophrenic patients. Nevertheless, dopamine receptors have remained of interest for schizophrenia research (Table 1). Advances in molecular biology permitted the identification of new subtypes of dopamine receptor. Two further ‘D2-like’ dopamine receptors, D3 and D4, attracted interest as potential drug targets, and the D3 site remains under-researched as a potentially important site of antipsychotic action. Initial excitement over the selectivity of clozapine for D4 receptors and their possible over-expression in schizophrenia was short-lived, however, and specific D4 antagonists are not antipsychotic.
D2 receptors; antipsychotic drugs compete with, and thereby decrease, ligand binding to these receptors. Thus it is possible to obtain an in vivo measure of receptor occupancy by drugs. This methodology has demonstrated that, at effective clinical doses, the classical antipsychotics occupy at least 70% of D2 receptors. Drug doses that induce extrapyramidal side effects are associated with higher (>80%) occupancy. However, the partial D2 agonist aripiprazole has very high occupancy without inducing extrapyramidal side effects, whereas clozapine, and now some newer antipsychotics, are found to be clinically effective at far lower levels of receptor occupancy. These findings imply differences in receptor mechanisms between classical and some atypical drugs. Such an interpretation has been reinforced by further PET and SPECT findings indicating regional differences in receptor occupancy of atypical antipsychotic drugs.2 These approaches are also being used to study occupancy at other receptors, including the 5-HT receptor subtypes, involved in atypical antipsychotic action. Synaptic dopamine function There is some evidence for changes in pre-synaptic dopamine, although these are likely to be secondary to changes in other neuronal systems, rather than a primary feature of the disease pathology. The greatest impetus in support of the dopamine hypothesis in the past few years has been the work emerging from in vivo neuro-imaging studies of dopamine release. PET imaging techniques can employ certain radioactive D2 antagonist drugs to provide an indirect indicator of synaptic dopamine levels. The extent by which such radioligands bind to the receptor will diminish with increasing amounts of competing dopamine, thus providing a relative measure of dopamine release in the synapse. Using this technique, a greater release of dopamine in the striatum is seen in schizophrenic subjects, relative to controls, following amphetamine administration (Figure 1).3 This process can be modelled by administration of
Understanding drug mechanisms In addition to permitting the measurement of brain receptor densities in vivo, neurochemical imaging using PET and SPECT has contributed enormously to our understanding of antipsychotic drug mechanisms. These techniques can measure the binding of radioactive ligands to a variety of sites, including dopamine
PSYCHIATRY 7:10
Pathway
426
© 2008 Elsevier Ltd. All rights reserved.
Research aspects
The effect of amphetamine administration on dopamine release Normal
Amphetaminestimulated dopamine release
Schizophrenia
Dopamine
Radio-labelled antagonist D2 receptor Increased dopamine in synapse results in greater dopamine occupancy of D2 receptors Figure 1
the psychotogenic anaesthetic ketamine, which also increases the release of dopamine; in both cases dopamine release appears to be proportional to the severity of psychosis. As ketamine is an antagonist at glutamate receptors, this might indicate that disturbed glutamate neurotransmission could underlie the findings in schizophrenia. However, these findings do not inevitably reflect disease pathology. In addition to psychosis, stress and nicotine can increase dopamine release and may both be experienced to a greater extent by schizophrenic patients. It is also far from clear whether these changes in dopamine function occur in the other brain regions that are more strongly implicated in the pathophysiology and pathology of the disease (Table 1). Nevertheless, one interpretation is that the control of dopamine release is disinhibited in schizophrenia, and that this effect may reflect a disturbance of glutamatergic neurotransmitter function. A further potential involvement of dopamine in schizophrenia is in the negative and cognitive symptoms. Dysfunction in the frontal cortex is likely to contribute to these symptoms and, on the basis of observations of an inverse relationship between the activities of cortical and striatal dopamine systems, it has been proposed that there is a hypofunction of dopamine in the frontal cortex in schizophrenia. Some supporting evidence has emerged from studies of catechol-O-methyltransferase (COMT),4 an enzyme involved in the metabolism of dopamine and noradrenaline. COMT can influence synaptic dopamine in the cortex, where increased dopamine concentrations – through effects at D1 receptors – may be associated with improved cognitive function. COMT has a common genetic polymorphism, whereby one allele codes for a less stable form of the enzyme, resulting in greater cortical dopamine concentrations. An association, albeit weak, between the COMT polymorphism and schizophrenia has been reported, where a relative overrepresentation of the higher-activity COMT form implicates cortical dopamine hypofunction as a disease risk factor. This focus on neuronal activity in the cortex also emphasizes the possible importance of disturbances in the intrinsic cortical neurons, containing GABA or glutamate, in the pathology of schizophrenia.5,6
PSYCHIATRY 7:10
GABA and glutamate – correlates of neuronal pathology GABA and glutamate are, respectively, the most common inhibitory and excitatory neurotransmitters in the brain. It is thus hardly surprising that they have both been substantially implicated in the widely investigated, yet subtle and poorly defined, neuronal pathology of schizophrenia.6 Glutamate dysfunction – models and consequences The evidence for disturbances in glutamate systems in the brain in schizophrenia is substantial. It includes the psychosis associated with administration of drugs such as phencyclidine (PCP) and ketamine, brought about by their blockade of the N-methyld-aspartate (NMDA) subtype of glutamate receptor. PCP’s psychotogenic effects include the development of negative as well as positive symptoms, providing a better model of schizophrenia than the primarily positive psychotic syndrome induced by (dopamine-releasing) amphetamine. The blockade of NMDA receptors by PCP or ketamine can have longer-term neurotoxic effects, a process that has led to an important hypothesis relating to a postulated neurodegeneration underlying progressive cognitive dysfunction in schizophrenia.7 Interestingly, this pathology is thought to be mediated by a hypofunction of GABAergic neurons, on which NMDA receptors can be found, resulting in disinhibition, and hence (toxic) overactivity of downstream neurons. GABAergic neuronal deficits There is much evidence for cortical and hippocampal losses of GABA-containing neurons. Along with morphological studies, a variety of different immunochemical markers of these interneurons, including co-existing neuropeptides, calcium-binding proteins, and the GABA-synthesizing enzyme glutamate decarboxylase (GAD), have all indicated selective deficits of subtypes of GABAergic neurons in the cortex and hippocampus.8 A recent analysis of 100 neurochemical investigations on a series of postmortem brain tissues demonstrated that the strongest findings in schizophrenia (and also, interestingly, in bipolar disorder) were frontal cortical or hippocampal deficits in parvalbumin, reelin, 427
© 2008 Elsevier Ltd. All rights reserved.
Research aspects
and GAD, all associated with subtypes of GABAergic neurons.9 A loss of these neurons may well have consequences equivalent to the long-term effects of NMDA receptor blockade described above, in which a further neuronal degeneration might result. Thus, the cortical GABAergic deficit in schizophrenia could well be an initial deficit, perhaps of neurodevelopmental origin, that subsequently results in a further, progressive, glutamatergic neuronal loss (Figure 2).
N-acetylaspartate – monitoring neuronal dysfunction in vivo N-acetylaspartate (NAA) is found in relatively high concentration in the brain and is one of the few substances that can reliably be identified there in vivo using proton MRS. The function of brain NAA is unclear, although it is considered to be a marker of neuronal integrity. It is a metabolite of N-acetyl-aspartylglutamate, which is found in neurons and is active at glutamate receptors. There have been many MRS investigations identifying NAA deficits in vivo in schizophrenia; these include cortical losses that correlate with PET measures of the increased dopamine release in the striatum, supporting the interpretation that dopaminergic hyperfunction might reflect disturbed corticostriatal innervation. There are also indications that cortical NAA losses correlate with disease duration, indicative of a degenerative process.11 As magnetic resonance imaging (MRI) technology has developed in sensitivity and resolution, MRS is being applied to determining other important molecules in the living brain, among them the individual components of the Glx complex: glutamate, glutamine, and GABA, as well as myo-inositol, considered a glial marker. Neuronal deficits may be responsible for other reported changes in schizophrenia. Decreases in some transmitter receptor proteins, such as 5-HT2A and the α7 nicotinic subunit, may reflect deficits of GABAergic cells on which these receptors are found, whereas other abnormalities may relate to compensatory up- or down-regulation of receptors, or differences in synaptic density. In addition, the artefactual influence of drug treatment on such findings cannot always be ruled out.
Glutamatergic abnormalities Changes have been observed in markers of glutamatergic neurotransmission in post-mortem studies. In general, these have indicated deficits of glutamate systems in the temporal cortex, medial temporal lobe, and striatal regions in schizophrenia, with losses of markers of glutamatergic terminals and increases, presumably compensatory, in NMDA receptors.6 These abnormalities point to deficits of cortico-subcortical innervation that may underlie cognitive dysfunction and negative features, as well as neuronal deficits (e.g. in the hippocampus) involved in other cognitive disturbances. The findings in frontal cortex are less clear; there are indications of an increase in glutamatergic synaptic density which is corroborated by some MRS measurements of glutamate in vivo. The observation that a polymorphism in a gene for a glutamate receptor (mGlu3) is a risk factor for schizophrenia, and that at least three other risk genes (DAAO, G72, and NRG1) can influence NMDA receptor function, further underlines the importance of glutamate neurotransmitter function in the disease.10
A speculative view of the role of neurotransmitter pathology in the natural history of schizophrenia
Neurochemical abnormalities of synaptic and membrane function
Genetic risk factors Birth
Environmental risk factors
2 years
Indications of delayed or disturbed neuronal development
GABAergic neuronal damage
6 years
Environmental risk factors Onset of psychosis
Given the complexity and variety of chemical and physiological processes associated with neuronal function, the opportunities to investigate these processes in schizophrenia are legion and impossible to review comprehensively. Nevertheless, reports relating to some neurochemical mediators of neuronal function have attracted particular interest. Abnormal amounts and functioning of certain subtypes of the G-proteins that mediate the effects of many receptors have been found in schizophrenia12; the schizophrenia risk gene RGS4 codes for a protein that regulates G-protein signalling. Interaction between G-proteins and their receptors may be disrupted by disturbances in the cell membranes in which the receptors are found; there is substantial evidence for imbalances in membrane phospholipids, as demonstrated by phosphorus-31 MRS of the brain in vivo.13 However, although it is tempting to implicate primary disturbances of lipid metabolism in schizophrenia that may respond to dietary supplementation, a variety of other factors including inadequate diet and substance abuse may contribute to artefactual findings. Recent findings in the molecular genetics of schizophrenia have implicated, as risk factors, genes involved in synaptic structure and function. This particularly includes, but is not restricted to, the genes influencing glutamate neurotransmission mentioned above.10,14 It is now emerging that some of these genetic risk factors have identifiable consequences in brain neurochemistry,14 as does COMT. The ‘risk allele’ can result in a neurochemical pathology, often with dysfunction of the gene product that may contribute to schizophrenia; NRG115 and mGluR316 provide two examples.
Disinhibition of glutamatergic neurons
20 years
Progressive glutamatergic neuronal damage
Cognitive decline
Increased subcortical dopamine function
60 years
Figure 2
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Research aspects
5 Winterer G, Weinberger DR. Genes, dopamine and cortical signal-tonoise ratio in schizophrenia. Trends Neurosci 2004; 27: 683–90. 6 Reynolds GP, Harte MK. The neuronal pathology of schizophrenia: molecules and mechanisms. Biochem Soc Trans 2007; 35: 433–36. 7 Olney JW, Newcomer JW, Farber NB. NMDA receptor hypofunction model of schizophrenia. J Psychiatr Res 1999; 33: 523–33. 8 Benes FM, Berretta S. GABAergic interneurons: implications for understanding schizophrenia and bipolar disorder. Neuropsychopharmacology 2001; 25: 1–27. 9 Torrey EF, Barci BM, Webster MJ, et al. Neurochemical markers for schizophrenia, bipolar disorder, and major depression in postmortem brains. Biol Psychiatry 2005; 57: 252–60. 10 Harrison PJ, Owen MJ. Genes for schizophrenia? Recent findings and their pathophysiological implications. Lancet 2003; 361: 417–19. 11 Molina V, Sánchez J, Reig S, et al. N-acetyl-aspartate levels in the dorsolateral prefrontal cortex in the early years of schizophrenia are inversely related to disease duration. Schizophr Res 2005; 73: 209–19. 12 Catapano LA, Manji HK. G protein-coupled receptors in major psychiatric disorders. Biochim Biophys Acta 2007; 1768: 976–93. 13 Smesny S, Rosburg T, Nenadic I, et al. Metabolic mapping using 2D 31P-MR spectroscopy reveals frontal and thalamic metabolic abnormalities in schizophrenia. Neuroimage 2007; 35: 729–37. 14 Weinberger DR. Genetic mechanisms of psychosis: in vivo and postmortem genomics. Clin Ther 2005; 27(Suppl. A): S8–15. 15 Harrison PJ, Law AJ. Neuregulin 1 and schizophrenia: genetics, gene expression, and neurobiology. Biol Psychiatry 2006; 60: 132–40. 16 Egan MF, Straub RE, Goldberg TE, et al. Variation in GRM3 affects cognition, prefrontal glutamate, and risk for schizophrenia. Proc Natl Acad Sci U S A 2004; 101: 12604–09. 17 Patil ST, Zhang L, Martenyi F, et al. Activation of mGlu2/3 receptors as a new approach to treat schizophrenia: a randomized phase 2 clinical trial. Nat Med 2007; 13: 1102–107.
Neurochemistry: linking pathology and drug treatment Neurochemistry is at the interface between genetics, brain pathology, and pharmacology. The one mechanism shared by current antipsychotic drugs is their action at dopamine D2 receptors, although how this ameliorates (some of) the symptoms of schizophrenia remains elusive. Current understanding of the regional specificity of neurochemical abnormalities, most notably those associated with GABAergic and glutamatergic neurotransmission, provides strong indications of the neuronal pathology underlying these symptoms. As there are correlates between these pathologies and abnormal dopamine release, which are attenuated by D2 antagonists, neurochemistry is beginning to bridge the gap between neuronal pathology and pharmacotherapeutic mechanisms. The current challenge is to ensure that our developing understanding of the neurochemical pathology underlying negative and cognitive symptoms, in which dopamine may not have a central role, translates into more effective treatments for these features of schizophrenia. The recent development of a potential antipsychotic targeting mGlu2/3 receptors may be a first step in this direction.17 ◆
References 1 Reynolds GP. Receptor mechanisms on the treatment of schizophrenia. J Psychopharmacol 2004; 18: 340–45. 2 Bressan RA, Erlandsson K, Jones HM, et al. Is regionally selective D2/D3 dopamine occupancy sufficient for atypical antipsychotic effect? An in vivo quantitative [123I]epidepride SPET study of amisulpride-treated patients. Am J Psychiatry 2003; 160: 1413–20. 3 Laruelle M, Abi-Dargham A. Dopamine as the wind of the psychotic fire: new evidence from brain imaging studies. J Psychopharmacol 1999; 13: 358–71. 4 Egan MF, Goldberg TE, Kolachana BS, et al. Effect of COMT Val108/158Met genotype on frontal lobe function and risk for schizophrenia. Proc Natl Acad Sci U S A 2001; 98: 6917–22.
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© 2008 Elsevier Ltd. All rights reserved.
The neurochemistry of schizophrenia
What’s new? • The genes implicated as risk factors in schizophrenia are increasingly found to influence neuronal and synaptic neurochemistry
Gavin P Reynolds
• Research has focused particularly on glutamatergic systems, and drugs are now in development that directly act on glutamate neurotransmission. If they prove successful in the clinic, this integration of genetics, neurochemistry, and pharmacology may have brought about a move away from dopamine antagonism in antipsychotic treatment
Abstract For more than 30 years much of the focus of neurochemical research in schizophrenia has been on the dopamine hypothesis, although serotonin systems may also be dysfunctional. Certainly, the primary action of antipsychotic drugs is to diminish dopamine D2 receptor-mediated neurotransmission. Although there is little indication of primary disturbances in dopamine (or serotonin) neurotransmission in the schizophrenia, recent functional neuro-imaging studies have demonstrated an increase in stimulated release of dopamine in the brain of patients with schizophrenia. It seems likely that this neurochemical correlate of positive symptoms might be secondary to disturbances in other neurotransmitter systems. Evidence from in vivo imaging and post-mortem studies of the brain in schizophrenia, as well as from experimental models, points to deficits of γ-aminobutyric acid (GABA)-containing neurons, and dysfunction of glutamate-containing neurons, in the cortex and elsewhere. Such regionally specific neuronal abnormalities probably underlie negative features and cognitive deficits, as well as contributing to a disinhibition of subcortical dopamine. Experimental models suggest that GABAergic deficits, perhaps of developmental origin, could result in progressive damage to other neuronal systems. Several of the recently identified genetic risk factors for schizophrenia also influence neurotransmitter and synaptic function, with some convergence on glutamate. This is providing new targets for antipsychotic drug treatment.
observations of neurochemical disturbances in living brains have been obtained using magnetic resonance spectroscopy (MRS), and positron and single-photon emission (computed) tomography (PET and SPET/SPECT respectively). The application of molecular genetics has also provided clues as to the underlying aetiologies and pathological processes in schizophrenia. Despite these technological developments, there are nevertheless inconsistencies in neurochemical findings in the disease. These problems may be due in part to the effects of drug treatment, as well as differences between patient cohorts (diagnosis in schizophrenia has not always been reliable, and symptom profile can vary enormously between individuals). The absence of a single core syndrome, and the multiplicity of proposed aetiological factors, indicates that schizophrenia is likely to be a complex disorder in which brain pathology may well differ between individuals owing to differences in pathogenic mechanisms. Early hypotheses Initial neurochemical hypotheses of brain dysfunction in schizophrenia assumed a disturbance of brain biochemistry. This approach originated in the observation that psychosis in humans, and equivalent bizarre behaviours in animals, could be induced by certain ‘psychotogenic’ drugs. It was assumed that the metabolic and pharmacological effects of these drugs might reflect the underlying biochemical abnormality in schizophrenia. Two hypotheses of the early 1950s involved excessive transmethylation and serotonin deficiency. It was suggested that neuro-active drugs with similarities to neurotransmitters, such as mescaline and dimethyltryptamine, were proposed to be formed by an overactive methylation process. Structurally similar psychotogenic compounds, such as lysergic acid diethylamide (LSD), were found to have effects via serotonin (5-hydroxytryptamine; 5-HT) receptors, and hence a deficiency of 5-HT neurotransmission was proposed. In retrospect, it is easy to identify limitations of these hypotheses, one being the inadequacy of the drug-induced ‘model psychosis’ in which distortions of reality and visual hallucinations are a major feature, rather than the auditory hallucinations and delusions more commonly seen in schizophrenia.
Keywords antipsychotic drugs; dopamine; dopamine receptors; GABA; glutamate; N-acetylaspartate; neuro-imaging; neurotransmitter
Methods and models The first approaches to understanding the pathophysiology of psychiatric disorders were chemical investigations of body fluids and, occasionally, brain tissue. These were under way long before the emergence of modern psychiatry, and for over 200 years have reflected methodological development, starting with the application of quantitative chemical analysis. The neurochemical study of the brain in schizophrenia depends on technological advance as much now as it did two centuries ago. Recently, exciting
Gavin P Reynolds PhD is Professor of Neuroscience in the Division of Psychiatry and Neuroscience at Queen’s University, Belfast, Northern Ireland, UK, and President of the British Association for Psychopharmacology. His main research interests are the pathology of neurotransmitter systems in schizophrenia and the mechanisms and pharmacogenetics of antipsychotic drug action. Conflicts of interest: none declared.
PSYCHIATRY 7:10
5-HT neurotransmission Nevertheless there remains an interest in the role of 5-HT in schizophrenia. Circumstantial support is provided by the atypical 425
© 2008 Elsevier Ltd. All rights reserved.
Research aspects
antipsychotic drugs, which all exhibit antagonism for the 5-HT2A receptor subtype; this provides a putative mechanism, not only for their low propensity to cause extrapyramidal side effects, but also for some efficacy in ameliorating the negative features of schizophrenia.1 More direct involvement is demonstrated by post-mortem and some neuro-imaging studies that have shown decrements in 5-HT2A receptors, and increases in 5-HT1A receptors, in the brain in schizophrenia. This is likely to reflect disturbances in 5-HT neurotransmission, although it is far from clear how these neurochemical abnormalities might originate in, or be secondary to, other neuronal pathologies such as those of γaminobutyric acid (GABA) and glutamate systems (see below). Nevertheless, changes in 5-HT function are perhaps unsurprising, given its central role in depression, a frequent symptom in schizophrenia, and the sensitivity of 5-HT systems to stress, an inevitable experience for many patients. Other neurochemical hypotheses have been proposed, based on aberrant neurotransmitter function (e.g. implicating noradrenaline or enkephalin), or metabolism, such as diminished monoamine oxidase activity. However, none of these has survived careful scrutiny and testing.
Dopamine pathways in the brain Physiological effects
Nigrostriatal
Involved in motor control; blockade of striatal dopamine D2 receptors produces extrapyramidal side effects; also involved in some cognitive circuits Blockade of limbic D2 receptors may alleviate positive symptoms of schizophrenia; limbic dopamine is also involved in reward and addiction Dopaminergic mechanisms in the frontal cortex modulate cognitive function, primarily via D1 receptors; COMT activity influences cortical synaptic dopamine Projection from hypothalamus controls pituitary hormonal secretion; blockade of D2 receptors disinhibits secretion of prolactin
Mesolimbic
Mesocortical
Tubero-infundibular
Table 1
The dopamine hypothesis Dopamine receptors What has survived the test of time, albeit with some elaboration, is the dopamine hypothesis of schizophrenia. This was first based on the ability of amphetamine, which stimulates dopamine release, to induce a psychosis with schizophreniform features. Further support came from the finding that almost all antipsychotic drugs are effective antagonists of the D2 subtype(s) of dopamine receptor, this antagonism correlating closely with clinical dosage. Subsequently, the finding of more D2 receptors in post-mortem brain from schizophrenic patients led to the hypothesis that the increase in D2 receptors resulted in the positive symptoms of schizophrenia. However, as an up-regulation of D2 receptors is seen in animals after chronic administration of antipsychotic drugs, it seemed likely that the increase in schizophrenia is a consequence of drug treatment and unrelated to the disease process. Most post-mortem and imaging studies now conclude that D2 receptors are not increased in drug-free schizophrenic patients. Nevertheless, dopamine receptors have remained of interest for schizophrenia research (Table 1). Advances in molecular biology permitted the identification of new subtypes of dopamine receptor. Two further ‘D2-like’ dopamine receptors, D3 and D4, attracted interest as potential drug targets, and the D3 site remains under-researched as a potentially important site of antipsychotic action. Initial excitement over the selectivity of clozapine for D4 receptors and their possible over-expression in schizophrenia was short-lived, however, and specific D4 antagonists are not antipsychotic.
D2 receptors; antipsychotic drugs compete with, and thereby decrease, ligand binding to these receptors. Thus it is possible to obtain an in vivo measure of receptor occupancy by drugs. This methodology has demonstrated that, at effective clinical doses, the classical antipsychotics occupy at least 70% of D2 receptors. Drug doses that induce extrapyramidal side effects are associated with higher (>80%) occupancy. However, the partial D2 agonist aripiprazole has very high occupancy without inducing extrapyramidal side effects, whereas clozapine, and now some newer antipsychotics, are found to be clinically effective at far lower levels of receptor occupancy. These findings imply differences in receptor mechanisms between classical and some atypical drugs. Such an interpretation has been reinforced by further PET and SPECT findings indicating regional differences in receptor occupancy of atypical antipsychotic drugs.2 These approaches are also being used to study occupancy at other receptors, including the 5-HT receptor subtypes, involved in atypical antipsychotic action. Synaptic dopamine function There is some evidence for changes in pre-synaptic dopamine, although these are likely to be secondary to changes in other neuronal systems, rather than a primary feature of the disease pathology. The greatest impetus in support of the dopamine hypothesis in the past few years has been the work emerging from in vivo neuro-imaging studies of dopamine release. PET imaging techniques can employ certain radioactive D2 antagonist drugs to provide an indirect indicator of synaptic dopamine levels. The extent by which such radioligands bind to the receptor will diminish with increasing amounts of competing dopamine, thus providing a relative measure of dopamine release in the synapse. Using this technique, a greater release of dopamine in the striatum is seen in schizophrenic subjects, relative to controls, following amphetamine administration (Figure 1).3 This process can be modelled by administration of
Understanding drug mechanisms In addition to permitting the measurement of brain receptor densities in vivo, neurochemical imaging using PET and SPECT has contributed enormously to our understanding of antipsychotic drug mechanisms. These techniques can measure the binding of radioactive ligands to a variety of sites, including dopamine
PSYCHIATRY 7:10
Pathway
426
© 2008 Elsevier Ltd. All rights reserved.
Research aspects
The effect of amphetamine administration on dopamine release Normal
Amphetaminestimulated dopamine release
Schizophrenia
Dopamine
Radio-labelled antagonist D2 receptor Increased dopamine in synapse results in greater dopamine occupancy of D2 receptors Figure 1
the psychotogenic anaesthetic ketamine, which also increases the release of dopamine; in both cases dopamine release appears to be proportional to the severity of psychosis. As ketamine is an antagonist at glutamate receptors, this might indicate that disturbed glutamate neurotransmission could underlie the findings in schizophrenia. However, these findings do not inevitably reflect disease pathology. In addition to psychosis, stress and nicotine can increase dopamine release and may both be experienced to a greater extent by schizophrenic patients. It is also far from clear whether these changes in dopamine function occur in the other brain regions that are more strongly implicated in the pathophysiology and pathology of the disease (Table 1). Nevertheless, one interpretation is that the control of dopamine release is disinhibited in schizophrenia, and that this effect may reflect a disturbance of glutamatergic neurotransmitter function. A further potential involvement of dopamine in schizophrenia is in the negative and cognitive symptoms. Dysfunction in the frontal cortex is likely to contribute to these symptoms and, on the basis of observations of an inverse relationship between the activities of cortical and striatal dopamine systems, it has been proposed that there is a hypofunction of dopamine in the frontal cortex in schizophrenia. Some supporting evidence has emerged from studies of catechol-O-methyltransferase (COMT),4 an enzyme involved in the metabolism of dopamine and noradrenaline. COMT can influence synaptic dopamine in the cortex, where increased dopamine concentrations – through effects at D1 receptors – may be associated with improved cognitive function. COMT has a common genetic polymorphism, whereby one allele codes for a less stable form of the enzyme, resulting in greater cortical dopamine concentrations. An association, albeit weak, between the COMT polymorphism and schizophrenia has been reported, where a relative overrepresentation of the higher-activity COMT form implicates cortical dopamine hypofunction as a disease risk factor. This focus on neuronal activity in the cortex also emphasizes the possible importance of disturbances in the intrinsic cortical neurons, containing GABA or glutamate, in the pathology of schizophrenia.5,6
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GABA and glutamate – correlates of neuronal pathology GABA and glutamate are, respectively, the most common inhibitory and excitatory neurotransmitters in the brain. It is thus hardly surprising that they have both been substantially implicated in the widely investigated, yet subtle and poorly defined, neuronal pathology of schizophrenia.6 Glutamate dysfunction – models and consequences The evidence for disturbances in glutamate systems in the brain in schizophrenia is substantial. It includes the psychosis associated with administration of drugs such as phencyclidine (PCP) and ketamine, brought about by their blockade of the N-methyld-aspartate (NMDA) subtype of glutamate receptor. PCP’s psychotogenic effects include the development of negative as well as positive symptoms, providing a better model of schizophrenia than the primarily positive psychotic syndrome induced by (dopamine-releasing) amphetamine. The blockade of NMDA receptors by PCP or ketamine can have longer-term neurotoxic effects, a process that has led to an important hypothesis relating to a postulated neurodegeneration underlying progressive cognitive dysfunction in schizophrenia.7 Interestingly, this pathology is thought to be mediated by a hypofunction of GABAergic neurons, on which NMDA receptors can be found, resulting in disinhibition, and hence (toxic) overactivity of downstream neurons. GABAergic neuronal deficits There is much evidence for cortical and hippocampal losses of GABA-containing neurons. Along with morphological studies, a variety of different immunochemical markers of these interneurons, including co-existing neuropeptides, calcium-binding proteins, and the GABA-synthesizing enzyme glutamate decarboxylase (GAD), have all indicated selective deficits of subtypes of GABAergic neurons in the cortex and hippocampus.8 A recent analysis of 100 neurochemical investigations on a series of postmortem brain tissues demonstrated that the strongest findings in schizophrenia (and also, interestingly, in bipolar disorder) were frontal cortical or hippocampal deficits in parvalbumin, reelin, 427
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Research aspects
and GAD, all associated with subtypes of GABAergic neurons.9 A loss of these neurons may well have consequences equivalent to the long-term effects of NMDA receptor blockade described above, in which a further neuronal degeneration might result. Thus, the cortical GABAergic deficit in schizophrenia could well be an initial deficit, perhaps of neurodevelopmental origin, that subsequently results in a further, progressive, glutamatergic neuronal loss (Figure 2).
N-acetylaspartate – monitoring neuronal dysfunction in vivo N-acetylaspartate (NAA) is found in relatively high concentration in the brain and is one of the few substances that can reliably be identified there in vivo using proton MRS. The function of brain NAA is unclear, although it is considered to be a marker of neuronal integrity. It is a metabolite of N-acetyl-aspartylglutamate, which is found in neurons and is active at glutamate receptors. There have been many MRS investigations identifying NAA deficits in vivo in schizophrenia; these include cortical losses that correlate with PET measures of the increased dopamine release in the striatum, supporting the interpretation that dopaminergic hyperfunction might reflect disturbed corticostriatal innervation. There are also indications that cortical NAA losses correlate with disease duration, indicative of a degenerative process.11 As magnetic resonance imaging (MRI) technology has developed in sensitivity and resolution, MRS is being applied to determining other important molecules in the living brain, among them the individual components of the Glx complex: glutamate, glutamine, and GABA, as well as myo-inositol, considered a glial marker. Neuronal deficits may be responsible for other reported changes in schizophrenia. Decreases in some transmitter receptor proteins, such as 5-HT2A and the α7 nicotinic subunit, may reflect deficits of GABAergic cells on which these receptors are found, whereas other abnormalities may relate to compensatory up- or down-regulation of receptors, or differences in synaptic density. In addition, the artefactual influence of drug treatment on such findings cannot always be ruled out.
Glutamatergic abnormalities Changes have been observed in markers of glutamatergic neurotransmission in post-mortem studies. In general, these have indicated deficits of glutamate systems in the temporal cortex, medial temporal lobe, and striatal regions in schizophrenia, with losses of markers of glutamatergic terminals and increases, presumably compensatory, in NMDA receptors.6 These abnormalities point to deficits of cortico-subcortical innervation that may underlie cognitive dysfunction and negative features, as well as neuronal deficits (e.g. in the hippocampus) involved in other cognitive disturbances. The findings in frontal cortex are less clear; there are indications of an increase in glutamatergic synaptic density which is corroborated by some MRS measurements of glutamate in vivo. The observation that a polymorphism in a gene for a glutamate receptor (mGlu3) is a risk factor for schizophrenia, and that at least three other risk genes (DAAO, G72, and NRG1) can influence NMDA receptor function, further underlines the importance of glutamate neurotransmitter function in the disease.10
A speculative view of the role of neurotransmitter pathology in the natural history of schizophrenia
Neurochemical abnormalities of synaptic and membrane function
Genetic risk factors Birth
Environmental risk factors
2 years
Indications of delayed or disturbed neuronal development
GABAergic neuronal damage
6 years
Environmental risk factors Onset of psychosis
Given the complexity and variety of chemical and physiological processes associated with neuronal function, the opportunities to investigate these processes in schizophrenia are legion and impossible to review comprehensively. Nevertheless, reports relating to some neurochemical mediators of neuronal function have attracted particular interest. Abnormal amounts and functioning of certain subtypes of the G-proteins that mediate the effects of many receptors have been found in schizophrenia12; the schizophrenia risk gene RGS4 codes for a protein that regulates G-protein signalling. Interaction between G-proteins and their receptors may be disrupted by disturbances in the cell membranes in which the receptors are found; there is substantial evidence for imbalances in membrane phospholipids, as demonstrated by phosphorus-31 MRS of the brain in vivo.13 However, although it is tempting to implicate primary disturbances of lipid metabolism in schizophrenia that may respond to dietary supplementation, a variety of other factors including inadequate diet and substance abuse may contribute to artefactual findings. Recent findings in the molecular genetics of schizophrenia have implicated, as risk factors, genes involved in synaptic structure and function. This particularly includes, but is not restricted to, the genes influencing glutamate neurotransmission mentioned above.10,14 It is now emerging that some of these genetic risk factors have identifiable consequences in brain neurochemistry,14 as does COMT. The ‘risk allele’ can result in a neurochemical pathology, often with dysfunction of the gene product that may contribute to schizophrenia; NRG115 and mGluR316 provide two examples.
Disinhibition of glutamatergic neurons
20 years
Progressive glutamatergic neuronal damage
Cognitive decline
Increased subcortical dopamine function
60 years
Figure 2
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5 Winterer G, Weinberger DR. Genes, dopamine and cortical signal-tonoise ratio in schizophrenia. Trends Neurosci 2004; 27: 683–90. 6 Reynolds GP, Harte MK. The neuronal pathology of schizophrenia: molecules and mechanisms. Biochem Soc Trans 2007; 35: 433–36. 7 Olney JW, Newcomer JW, Farber NB. NMDA receptor hypofunction model of schizophrenia. J Psychiatr Res 1999; 33: 523–33. 8 Benes FM, Berretta S. GABAergic interneurons: implications for understanding schizophrenia and bipolar disorder. Neuropsychopharmacology 2001; 25: 1–27. 9 Torrey EF, Barci BM, Webster MJ, et al. Neurochemical markers for schizophrenia, bipolar disorder, and major depression in postmortem brains. Biol Psychiatry 2005; 57: 252–60. 10 Harrison PJ, Owen MJ. Genes for schizophrenia? Recent findings and their pathophysiological implications. Lancet 2003; 361: 417–19. 11 Molina V, Sánchez J, Reig S, et al. N-acetyl-aspartate levels in the dorsolateral prefrontal cortex in the early years of schizophrenia are inversely related to disease duration. Schizophr Res 2005; 73: 209–19. 12 Catapano LA, Manji HK. G protein-coupled receptors in major psychiatric disorders. Biochim Biophys Acta 2007; 1768: 976–93. 13 Smesny S, Rosburg T, Nenadic I, et al. Metabolic mapping using 2D 31P-MR spectroscopy reveals frontal and thalamic metabolic abnormalities in schizophrenia. Neuroimage 2007; 35: 729–37. 14 Weinberger DR. Genetic mechanisms of psychosis: in vivo and postmortem genomics. Clin Ther 2005; 27(Suppl. A): S8–15. 15 Harrison PJ, Law AJ. Neuregulin 1 and schizophrenia: genetics, gene expression, and neurobiology. Biol Psychiatry 2006; 60: 132–40. 16 Egan MF, Straub RE, Goldberg TE, et al. Variation in GRM3 affects cognition, prefrontal glutamate, and risk for schizophrenia. Proc Natl Acad Sci U S A 2004; 101: 12604–09. 17 Patil ST, Zhang L, Martenyi F, et al. Activation of mGlu2/3 receptors as a new approach to treat schizophrenia: a randomized phase 2 clinical trial. Nat Med 2007; 13: 1102–107.
Neurochemistry: linking pathology and drug treatment Neurochemistry is at the interface between genetics, brain pathology, and pharmacology. The one mechanism shared by current antipsychotic drugs is their action at dopamine D2 receptors, although how this ameliorates (some of) the symptoms of schizophrenia remains elusive. Current understanding of the regional specificity of neurochemical abnormalities, most notably those associated with GABAergic and glutamatergic neurotransmission, provides strong indications of the neuronal pathology underlying these symptoms. As there are correlates between these pathologies and abnormal dopamine release, which are attenuated by D2 antagonists, neurochemistry is beginning to bridge the gap between neuronal pathology and pharmacotherapeutic mechanisms. The current challenge is to ensure that our developing understanding of the neurochemical pathology underlying negative and cognitive symptoms, in which dopamine may not have a central role, translates into more effective treatments for these features of schizophrenia. The recent development of a potential antipsychotic targeting mGlu2/3 receptors may be a first step in this direction.17 ◆
References 1 Reynolds GP. Receptor mechanisms on the treatment of schizophrenia. J Psychopharmacol 2004; 18: 340–45. 2 Bressan RA, Erlandsson K, Jones HM, et al. Is regionally selective D2/D3 dopamine occupancy sufficient for atypical antipsychotic effect? An in vivo quantitative [123I]epidepride SPET study of amisulpride-treated patients. Am J Psychiatry 2003; 160: 1413–20. 3 Laruelle M, Abi-Dargham A. Dopamine as the wind of the psychotic fire: new evidence from brain imaging studies. J Psychopharmacol 1999; 13: 358–71. 4 Egan MF, Goldberg TE, Kolachana BS, et al. Effect of COMT Val108/158Met genotype on frontal lobe function and risk for schizophrenia. Proc Natl Acad Sci U S A 2001; 98: 6917–22.
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